This invention relates to a method and a system for the determination or assessment of the number of somatic cells (or fragments thereof, the fragments to be understood to be included whenever somatic cells are mentioned in the following) in a milk or a milk product analyte material. The present invention relates to the assessment of somatic cells in milk product analyte such as, raw milk collected at cow side, raw milk collected during milking, bulk milk delivered by the dairy farmer, milk and milk products produced by dairies and milk samples being measured in central laboratories.
Determinations or assessments of the number of somatic cells in a milk or a milk product analyte have been performed by various methods. One of these methods is flow cytometry; instrument for performing flow cytometry is available, e.g., from Becton, Dickinson and Company, Franklin Lakes, USA.
For example, EP 0 556 971 describes a flow cytometric method for assessing the number of particles in a fluid. The fluid is passed by a sensor which detects light signal emitted from the sample. When the sensor detects a change in the light signal, a particle detection is triggered. The particle detection involves generating a pulse of excitation light and hence an intermittent light emission. Light emitted from the flow cell is then focused onto a CCD camera which can produce an image of the particle.
Flow cytometry requires rather elaborate and high cost equipment, partly because of the high accuracy of flow rate necessary to give reliable results, and partly because of the high sensitivity needed to detect the weak signals from the particles in question during the relative short period of time the particle is present in the detector.
Another example of flow cytometry is described in U.S. Pat. No. 5,428,451 wherein particles in a fluid are counted by passing the fluid through an optical cell and allowing an image of the particles to be projected onto an array of charge coupled devices (CCDs). Several arrays of CCDs may be arranged after each other to obtain several pictures of the cells during the flow through the flow cell.
Another known method for the determination of somatic cells in milk is based on the detection of signals from particles which are dispersed on the rim of a polished rotating disc, one such instrument being available from Foss Electric, Hillerxc3x8d, Denmark. The accuracy in the assessment of the number of particles using this method is dependent on the physical shape of the thin film of sample dispersed on the disk, and high sensitivity is needed to detect the weak signals from the particles in question in the course of the relative short period of time the particle is present in the detector.
One known method for the determination of somatic cells in milk is based on spreading a film of milk onto a ribbon-like film which is then analysed by the means of a microscope, cf. European patent 0 683 395"". This method appears to require a complex mechanical solution in order to work reliably.
Yet another method of assessing the number of particles in milk is described in WO 96/31764 wherein quantitative determination of particles in fluid is carried out by the use of an apparatus comprising an emitter set of light emitters in combination with a detector set of light detectors. Light scattering due to the particles in the sample gives rise to a plurality of signal paths between the emitter and detector sets and these data are gathered and analysed. Thereby information regarding the particle content is obtained, however distinction between the various particles in the same size distribution, such as distinction between cells and fat particles, is not possible.
Due to the relative high complexity and cost of the instruments used today, most of the assessments of the number of somatic cells in a milk or a milk product analyte are carried out in a laboratory where skilled operators operate the instruments.
The present invention offers substantial simplification of the assessment of the number of somatic cells in a milk or a milk product analyte material and therefore makes it possible for operators without any particular skill in this fields of technique to perform the assessment. In particular, the invention makes it possible to perform the assessment on the farm where the sample is taken, thus making the results of the assessment available for the user substantially immediately after the sample material has been collected.
The physical dimension of an instrument based on the present invention is also such that the instrument will be well suited for transport, thus making it possible for e.g. veterinarians to transport the instrument to or on a location where the analysis is needed. The principle of measurement of the present invention provides a major improvement in the assessment of DNA-containing particles, e.g. somatic cells in a milk or a milk product analyte material, compared to the methods hitherto used for this purpose.
This invention extends the capabilities of prior devices and methods to enable more simple and reliable assessment of biological particles in liquid analyte material. The properties which can be assessed are the number of somatic cells in a milk or a milk product analyte material.
At the same time, this invention allows these analyses to be carried out with the use of considerably smaller amounts of chemicals than normally required to do these analyses. These chemicals are often considered hazardous, either to humans and other living organisms or to the environment. Furthermore, this invention presents a solution which minimises the exposure of any hazardous sample or chemicals used for the analysis by either allowing the analysis to be performed in a closed flow system or by the use of a sealed and disposable sample compartment which contains all sample material and chemicals used for the assessment and allows safe transport of the sample and any chemicals.
The high cost as well as the mechanical complexity of the instruments hitherto used for the routine assessment of the number of somatic cells in a milk or a milk product analyte material have made the instruments impractical to use routinely under condition such as are normally present on dairy farms, on milk dairies, or in veterinary clinics. Such analyses are of great interest; for instance, a dairy farmer can monitor the somatic cell count or bacterial count of an individual animal in order to follow the course of clinical or subclinical mastitis or infection, and to control the cell count of the bulk milk delivered to the dairy, thereby minimising the use of antibiotics and preventing the economical penalty which is often a consequence when the cell count of bulk milk exceeds predefined limits.
This invention is particularly suited for the assessment of the number of somatic cells in milk from human, cow, goat, sheep, buffalo or other animal. In particular, this invention is suited for the assessment of the number of somatic cells in milk during milking by integrating the system with the milking equipment, either in-line where the measurement is taken substantially from the milking system and analysed by an instrument which is operated synchronised with the milking, or at-line where the sample is taken before, during or after milking and measured on an instrument in manual operation. In particular it is well suited to obtain an estimate of the number of somatic cells when the purpose of the analysis is to control the number of somatic cells in the bulk of milk delivered to the dairy, for instance by directing any milk which is found to have high cell count to a separate container or outlet.
Methods according to the invention are suited for the on-line or at-line assessment of the number of somatic cells in milk when the purpose is to establish information about the health status of animals, such as cows, goats, sheep or buffaloes, especially in connection with clinical or sub-clinical mastitis.
The method according to the invention is suited for the assessment of the number of somatic cells in milk when the objective of the analysis is to generate information used in a herd improvement scheme, or when the objective of the analysis is to obtain a quality parameter used in a payment scheme. These analyses are normally carried out in a central laboratory, by the use of complex instruments.
According to the invention, an array of detection elements can be utilised in combination with appropriate electronic components, to accomplish the assessment of somatic cells in a milk or a milk product analyte material by placing a portion of the analyte material in a sample compartment, the sample compartment in many embodiments of this invention being two windows of glass, or other transparent material, separated by a spacer with inlet and outlet which allows the sample to be replaced between measurements; in one embodiment, the sample compartment is a tube, substantially circular, or substantially elliptical in profile. The presence of somatic cells will normally cause the signal from a detection element to deviate from a normal level, e.g. a base-line level, either towards higher signal intensity or toward lower signal intensity, but for the sake of clarity, in the following it will be assumed that such deviation is toward higher signal intensity.
The present invention is based on the arrangement of the sample in such a manner that it extends over a xe2x80x9cwindowxe2x80x9d of a substantial area and detection of signals from the samples in the form of an xe2x80x9cimagexe2x80x9d on an array of detection elements, the array of detection elements comprising individual elements each of which is capable of sensing signals from a part of the sample window area, the array as a whole being capable of sensing signals from substantially all of the sample window area, or at least a well defined part of the sample window area.
As will appear from the following, the arrangement of the sample and the detection elements in this way will allow the determination of the number of the somatic cells per volume in a much more simple and economic manner, while retaining a high accuracy of the determination. Also, as will be explained in the following, the use of an array of detection elements xe2x80x9cobservingxe2x80x9d an exposed area of the sample makes it possible to use quite simple means for generating signals from the sample and quite simple and sensitive detection means.
Thus, an aspect of the invention can be expressed as a method for the assessment of the number of somatic cells in a volume of liquid milk or a milk product material, the method comprising arranging a sample of the liquid sample material in a sample compartment having a wall defining an exposing area, the wall allowing signals from the sample to pass through the wall and to be exposed to the exterior, forming an image of signals from the sample in the sample compartment on an array of detection elements, processing the image on said array of detection elements in such a manner that signals from said particles are identified as distinct from the sample background, and, based on the signals from said particles identified assessing the number of particles in a volume of said liquid sample material.
Expressed in another and more general way, this aspect of the invention relates to a method for the assessment of somatic cells in a milk or a milk product analyte material, comprising
arranging a volume of a liquid sample representing the analyte material in a sample compartment having a wall part defining an exposing area, the wall part allowing electromagnetic signals from the sample in the compartment to pass through the wall and to be exposed to the exterior,
exposing, onto an array of active detection elements, an at least one-dimensional spatial representation of electromagnetic signals having passed through the wall part from the sample in the sample compartment, the representation being one which is detectable as an intensity by individual active detection elements, under conditions which will permit processing of the intensities detected by the array of detection elements during the exposure in such a manner that representations of electromagnetic signals from the somatic cells are identified as distinct from representations of electromagnetic signals from background,
the size of the volume of the liquid sample being sufficiently large to permit the assessment of the number of somatic cells to fulfil a predetermined requirement to the statistical quality of the assessment based on substantially one exposure,
processing the intensities detected by the detection elements in such a manner that signals from the somatic cells are identified as distinct from background signals,
and correlating the results of the processing to the number of somatic cells in the liquid analyte material.
The liquid sample representing the analyte material may be a liquid sample consisting of the liquid analyte material per se (optionally and often preferably with added chemical substances facilitating the assessment, such as will be explained in the following), or it may be a sample which has been derived from the liquid analyte material by dilution, concentration, extraction, or other modification. In this connection it is, of course, normally essential that there is an unambiguous correlation between the volume of the liquid sample representing the liquid analyte material and the volume of the liquid analyte material in question, so that the necessary correlation to a concentration in the liquid anaiyte can be established.
Alternatively, but generally not preferred, particles isolated from a volume of a liquid sample representing the liquid analyte material may be the material from which the exposure onto the array of detection elements is made. This is the case, e.g., when a liquid sample representing the liquid analyte material has been filtered through a filter material, and the filter material with the retained particles, often after addition of chemicals facilitating the assessment, cf. below, such chemicals having been added before or normally after the filtration, is arranged in the domain from which the exposure is made, normally a sample compartment suited for housing the filter.
As mentioned above, the exposure of the electromagnetic signals having passed from the domain onto the array of detection elements will normally correspond to forming an xe2x80x9cimagexe2x80x9d of the domain (such as an exposing area of a wall part of a sample compartment) on a two-dimensional array of detection elements, but it is also possible to use a one-dimensional spatial representation, obtained by suitable optical means, in which case the array of detection elements need not be more than one-dimensional, such as a linear array of detection elements. In special embodiments, a linear array of detection elements can also be used for receiving a two-dimensional image of electromagnetic radiation, provided the area of each element is sufficient to receive signals from a sufficient volume to allow the quality requirements to the determination.
The intensity detected by the array of detection elements may be a charge built up due to the electromagnetic radiation, or it may be, e.g., the intensity of a current passing through the individual element as a result of the electromagnetic radiation.
The conditions of the exposure with respect to the various parameters involved, such as will be explained in greater detail below, are adapted so that the intensities detected by the array of detection elements can be processed, using suitable processing means, typically image processing means and methods, in such a manner that the intensities which have been detected as representations of electromagnetic signals from the biological particles are identified as distinct from representations of background signals.
The size of the volume of the liquid sample on which measurement is made, or from which the particles are isolated, should be sufficiently large to permit the determination of the concentration of somatic cells in such a way as to fulfil a predetermined requirement to the statistical quality of the assessment based on substantially one exposure. As will be explained in the following, it is a characteristic feature of the present invention that it permits the gathering of sufficient information in one exposure to allow a high statistical quality in spite of the fact that the assessment can be performed in an extremely simple manner. One reason for this is that the method of the invention is normally performed using much smaller enlargements of the image projected onto the array of detection elements than has hitherto been considered possible, and in some cases even reductions, in contrast to enlargements. For a number of applications, the degree of enlargement is just around 1:1, in contrast to most automated microscopy methods which use larger enlargements and several observations. In connection with the present invention, the term xe2x80x9csubstantially one exposurexe2x80x9d is to be understood as one exposure or in some cases just a few exposures such as two, three or four exposures, but the by far preferred embodiment is to use just one exposure, such as is made possible by the invention. The exposure may, under certain circumstances, be performed as a number of sub-exposures before the intensity detected by the array elements is processed, but this is normally not necessary or preferred.
The formation of an image of the sample on the array of detection elements may be performed by arranging the array of detection elements in close contact or substantially in close contact with the exterior of the exposing wall of the sample compartment, or by using an image-forming means, such as a lens comprising one or several elements, arranged in the light path between the exposing wall of the sample compartment and the array of detection elements.
The wall of the sample compartment defining an exposing area may be a flat or curved wall.
The sample in the sample compartment can be replaced by the means of a flow system which is driven by a pump or a pressurised gas, preferably air. In many embodiments of the present invention the flow in said flow system is controlled by one or more valves which can adjust the flow speed of the sample.
In many preferred embodiments of the present invention, the wall of the sample compartment is a plane wall, and the array of detection elements is an array extending in a plane parallel to the plane of the wall. However, dependent on the manner in which the image of the sample is formed on the array of detection elements, the configuration of each of the exposing wall and the array may be designed in many different ways, such as where both the exposing wall and the array are configured as sections of a circular cylinder, such as where the exposing wall is convex and the array is concave with substantially the same radius, whereby they can easily be brought in contact or in substantial contact with each other, or where both the exposing wall and the detection array are concave, and a lens is used for formation of the image of the sample on the array. Many other configurations are of course possible, such as where both the exposing wall and the array are sections of spheres, etc.
The sample compartment may be a chamber which can easily be removed from the instrument when a new sample or sample material is to be measured. Such removable sample compartment is preferably used for a limited number of measurements, and preferably only one. Apart from allowing a more simple mechanical construction of an instrument with the absence of any flow system, one advantage of such removable sample compartment is that it can contain the sample in a closed container before, during and after analysis, thus allowing more safe handling of hazardous material. In many embodiments of the present invention, a (removable) unit comprising such removable sample compartment can, prior to the introduction of any sample material, contain one or more components or devices used for chemical or physical modification of the sample prior to analysis.
Electronical devices or a computer equipped with suited software can be used to condition a signal which originates from any detection element used, preferably in such a way as to make the quantification of the signal from any detection element more reliable or less time consuming, for instance by converting one type of signal to another signal suited for processing, and/or by providing means for the amplification of the signal. Often it is preferred that the signal from any detection element is adjusted for any bias, and/or for any variation in sensitivity which might be present in the signals, this adjustment preferably being performed by taking into account information from neighbouring detection elements, or by using similar information from a previous measurement. Another useful property of such signal conditioning is the conversion of a substantially analogue signal to a digitised value which is better suited for further processing using a digital data processing system; such digitalisation could be a threshold-like activation of two or more output lines in such a way that the input level of any signal would cause a change of the status of these output lines, preferably in such a way that the level of the input signal could be estimated. A preferred method of digitalisation is one which allows the level of the input signal to be converted to a number according to the binary number system.
It is often preferred that the digital representation of the level of any input signal produces a substantially linear function, and in many embodiments of this invention it is preferred that the digital representation produces a substantially non-linear function, for instance a logarithmic function, such non-linear function being preferred when the dynamic range of the input level is high.
In some implementations of this invention, it is preferred to use a one dimensional array of detection elements, preferably included in one chip, the identification of a particle present in the sample which is measured being done by comparing the level of signal from each detection element with a predefined level, or preferably with a level which is estimated on the basis of the signals from neighbouring detection elements, preferably on the basis of the signals from previous measurements, and if a signal is found to be above this discriminating level it is assumed that a particle was present, and a counter is incremented accordingly. Furthermore, it is possible to detect the presence of two particles measured at once for instance by comparing the intensity of a signal to a known or determined limit in such a way that signals above such limit indicate the presence of two particles. More than one such limit can be used to identify any situation where three, four or more particles are present, or an empirical or theoretical relationship can be constructed between the total number of particles present, and the possibilities of signals from two or more particles being detected simultaneously by a detection element.
As mentioned above, it is often preferred that an optical system is used to focus any signal from the sample onto the detection elements, and further it is preferred that such focusing produces an image of a particle with an average size which is of about the same size as the detection elements used, and in some cases preferably smaller, such that the image of the entire particle is substantially within the boundaries of the detection element.
In other embodiments of this invention, similar to the one described above using a one dimensional array of detection elements, or two dimensional array of detection elements, it is preferred that an optical system is used to focus any signal from the sample onto the detection elements, in such a way as to produce an image which is of the same size as the detection elements used, and preferably greater, the method being used to identify the presence of a particle taking into account also the extension of the particle in the dimension along the row of detection elements as well as the height of the measured signal from each detection element. Such embodiment of this invention allows the estimation of some morphological properties of the particles which are measured, such as the size. Also under those conditions it is possible to detect the presence of two ore more particles which are focused on substantially the same detection elements, for instance by classifying the signal intensity.
It was surprisingly found that a one-dimensional array of detection elements, where the width of the array of detection elements was considerably greater than the height of each detection element, one such element being commercially available from Hamamatzu (S3902-128Q), could be used for the assessment of the number of particles and thus enabling the detection of signals from a greater volume of the sample in each scanning of the detection elements. Furthermore, it was discovered that the use of even a focusing device which distorts the dimensions of the image, relative to the original, in such a way that for instance the image of a circle has a shape which is similar to an ellipse, also gave similar advantage as the use of detection elements with great height, and further it was found that the combination of the above-mentioned detection elements and a distorting focusing device made it possible to obtain a useful assessment on a large detection volume.
The use of a series of one-dimensional arrays of detection elements, preferably incorporated in a single chip, is often found to be useful in the assessment of somatic particles in milk, one commercially available charge coupled device (CCD) being available from Sony (ICX 045 BL). Another array of detection elements suited for many embodiments of this invention is an image sensor based on CMOS technology which makes detection possible with the use of limited electrical effect, as well as offering on-chip integration with other CMOS based technologies such as signal condition and signal processing, one such having been demonstrated by Toshiba comprising 1318xc3x971030 elements each about 5.6 xcexcmxc3x975.6 xcexcm in size using only 30 mW effect in use.
The assessment of somatic cells in a sample can be performed by treating each line of such two dimensional array of detection elements in substantially the same manner as an array of one dimensional detection elements.
Some embodiments of this invention allow the simulation of high detection elements by the electronical or computational addition of information from two or more lines of detection elements into one array of information which is thereafter treated in substantially the same manner as a single one dimensional array of detection elements, thus allowing substantially simpler and less time consuming interpretation of the measured information.
In some embodiments of this invention, the assessment of the number of particles in a first line of detection elements is based on any results, such as position and/or intensities observed in a second line of detection elements already being processed, thus allowing the correction of signals which extend across two or more lines of detection elements.
The inclusion of a focusing device for the focusing of a signal from the sample onto the detection elements in such a manner as to maximise the collection angle, collection angle being defined as the full plane angle within which a signal is detected, is in many situations found to give improved condition for an assessment. Surprisingly it was found that such a wide collection angle, even to the extent that the objective used in the focusing distorted the aspect ratio of the image of any particle differently across the plane in which the detection elements were placed, or produced variation in the focusing across the sample being analysed, or reduction of the focusing quality, was applicable in the assessment of the number of particles.
It is possible to make the assessment of biological particles in a sample by using a calculation means, preferably a digital computer, such as one commercially available from Analogue Devices (ADSP 2101), equipped with storage capacity which can only store information in an amount substantially equivalent to a small fraction of the total number of detection elements, the assessment of the number of objects being based on substantially real time processing of data, preferably in such a way that the measured information from each detection element, or a line of detection elements, or two or more lines of detection elements, is used for the assessment, substantially without any delay, such as a delay caused by storing the measured information.
However, it is often preferred to store substantially all measured information by the use of a first calculation means, preferably a digital computer, before the processing of the information by a second calculation means, preferably a digital computer, and thus allowing the measured information to be processed at substantially the same rate as it is obtained, but with a substantial time delay between the measurement of any information and the processing of the same information; preferably, this is accomplished by using only one calculating means, preferably a digital computer, equipped with enough resources to accomplish the task.
When using a sample compartment for the analysis of more than one sample material, for instance when the sample is introduced by means of a flow system, it is often found that one or more of the particles of interest, or fractions of particles, adhere to the sample compartment in such a way that the flow used to replace the sample material is not capable of removing said adhering particles. Thus, if such an adhering particle is situated in a place which is exposed to the sensing device, it will be included in two or more observations although the sample has been substantially replaced between observations. In many embodiments of the present invention, the influence of such adhering particles on the observation can be substantially eliminated by combining two observations in such a way that the result from a first observation is adjusted by the result from a second observation, said second observation being one of many observations taken prior to said first observation or a combination of more than one of many observations taken prior to said first observation, preferably an observation taken substantially immediately prior to said first observation, said adjustment being a simple subtraction of said second observation from said first observation. The result of said adjustment then contains information where any objects present in said first observation have positive intensity, any object present in said second observation has negative intensity, and any object present in both first and second observation have substantially zero intensity. The task of any method used for the assessment of the number of objects is then to only treat those intensities which have substantially positive values. In a similar way it is possible to analyse the results of two or more observations taken from different samples from the same sample material by combining those observations as described above and subsequently to analyse both the positive and negative signals, for instance by treating all signals as being positive. In this way it is possible to analyse 2, 4, 6, 8 or more observations simultaneously, for instance in situations where the effort of analysing an observation is greater than the effort of making an observation.
In many preferred embodiments of this invention the sample material to be analysed has been modified or its chemical or physical properties substantially changed compared to the analyte material by either the addition of, or the removal of one or more components, or by introducing the sample to one or more chemical, mechanical or physical treatments prior to analysis. Preferably, the effect of any such alteration or modification is the enhancement of any measurable signal used for the analysis, or a suppression of any interfering phenomenon, or it has the effect of prolonging the working life of the sample.
It is often preferred that the signal which is detected is a photoluminescence signal, originating from a molecule, or a fraction of a molecule having fluorophor properties, naturally contained within or on the particle which is measured.
The particles which are to be detected are often xe2x80x9ccolouredxe2x80x9d with one or several molecules which bind to the particle, are retained within the particle, or otherwise interact with the particle, the effect of this xe2x80x9ccolouringxe2x80x9d being the enhancement of any signal from the particle, or being the direct source of a signal which thereby can be used to detect the particle.
In many aspects of the invention, the effect of the xe2x80x9ccolouringxe2x80x9d is to cause, or enhance, the attenuation of electromagnetic radiation such as visible light, or preferably to cause, or enhance, the emission of electromagnetic radiation such as chemiluminescence, or photoluminescence, e.g. fluorescence or phosphorescence, when excited with radiation which is substantially higher in energy than the emitted photoluminescence. One such xe2x80x9ccolouringxe2x80x9d is the addition of Ethidium Bromide (EtBr) to the sample, where EtBr interacts with DNA material present in the sample, giving rise to fluorescence at approximately 605 nm when excited with light at approximately 518 nm (Handbook of Fluorescent Probes and Research Chemicals, page 145). This makes it possible, in the combination with the appropriate set of optical filters, to count a DNA-containing particle such as a somatic cell where EtBr can interact with the DNA.
It was surprisingly found that it was possible to use concentrations of fluorophor which were substantially lower than those normally used in system, often less than 1/10th or 1/100 or even less than 1/1000; in particular, this is advantageous where added fluorophor exhibits relatively similar properties in free form as in bound form, with regard to intensity and wavelength characteristics. As expected, such condition inherently reduces any signal emitted from a coloured particle, but surprisingly it was found that the ratio of the signal intensity in bound form to free form shifted in favour of bound signals. In particular, it was found that a level of signal from fluorophor in free form in the sample which was comparable, and preferably less, in intensity to any random electronical signal (noise) and/or comparable in intensity to, and preferably less than, any other interfering signal was to be preferred.
It is often preferred that the liquid, in which particles to be measured are suspended, is substantially at stand-still, where stand-still is defined as the situation where at least a part of the image of a particle does not move any more than it is contained substantially within the boundary of the same detection elements during one measurement period. The stand-still situation is preferably such that at least a part of the image of a particle does not move any more than it is contained substantially within the boundary of the same detection element during at least two measurement periods, thus allowing the detection of any weak signals which might indicate the presence of a particle.
In other embodiments of this invention, normally less preferred, the liquid in which particles to be measured are suspended, is substantially moving during measurement, in such a way that at least a part of the image of a particle gives rise to signal in two or more adjacent detection elements during one measurement period, or in such a way that at least a part of the image of a particle gives rise to signal in two or more adjacent detection elements during at least two measurement periods.
The liquid in which particles to be measured are suspended can be moving in more than one direction during measurement, for instance by controlling two sources of force, which preferably can be applied perpendicular to each other, thus giving the opportunity to move the sample in a predefined pattern, which can be used to improve the performance of any image processing device used to analyse the measured signal.
It is possible to perform more than one measurement, thus allowing an even more accurate and/or sensitive assessment of the number of somatic cells, for instance by measuring the same portion of the sample more than once and combining the results in order to improve the signal to noise ratio, and/or to measure more than one portion of the sample in order to increase the total number of particles which are counted to reduce the error in the assessment, since the error in the particle count will normally follow count statistics where the relative error is expected to behave similar to one over the square root of number of counts. However, it is a characteristic feature of the present invention that its general character of detection based on a relatively large sample volume giving a large amount of information makes it possible to meet a predetermined statistical standard based on substantially one exposure.
In some embodiments of this invention, the number of measurements taken is defined by a real time estimate of the number of particles already counted thus performing relatively fewer measurements when the sample contains a high number of particles and relatively more measurements when the sample contains a low number of particles, preferably by defining an approximate lower limit for the total number of counted particles in such a way that an appropriate accuracy in the measurement is obtained.
It is possible to assess the biological particles in a relatively short time, thus allowing a high number of samples to be analysed per hour, often more than 300, 400, and even as many as 1000 or more analyses per hour. In many preferred embodiments of this invention an even higher number of analyses per hour is achieved by including more than one measurement unit, the measurement units working in parallel in a single instrument.
In many embodiments of this invention the signals which are detected are attenuation of electromagnetic radiation, for instance caused by absorption or scattering, and in many preferred embodiments of this invention the signals which are detected are emitted from the particles or the samples, for instance emission of photoluminescence (e.g. fluorescence and/or phosphorescence) or raman scatter, and in other embodiments of this invention the signals which are detected are caused by scatter.
Often more than one of the previously mentioned signals are detected simultaneously, thus allowing more accurate or sensitive assessment of the number of somatic cells, preferably by the use of more than one set of detection elements.
A monochromatic device can be used to separate electromagnetic radiation into one or more wavelength components before one or several of these wavelength components are transmitted onto the sample, either one at a time or more than one at a time, preferably when more than one wavelength component is transmitted onto the sample simultaneously the wavelength components are transmitted onto different portions of the sample, thus giving an opportunity to obtain qualitative as well as quantitative information about particles in the sample. This is in particular of interest when the sample contains particles which respond differently to different wavelength components.
Light which can be transmitted onto the sample can be focused by a focusing system, comprising one or more lenses each of which can comprise one or more elements. The effect of such a focusing system is often to increase the effective efficiency of the light source. As light source it is possible to use a thermal light source, such as a halogen lamp, or a gas lamp such as a xenon lamp, a light emitting diode, a laser or a laser diode. It is often preferred to use more than one light source for the purpose of increasing the flux of light onto the sample, for instance by using two or more light emitting diodes. It is also possible to use more than one light source where some of the light sources have different electromagnetic properties.
A monochromatic device can be used to separate electromagnetic radiation emitted from, or transmitted through the sample into one or more wavelength components before such electromagnetic radiation is detected by a detection element, either in such a way that one wavelength is measured at a time or in such a way that more than one wavelength components are measured at a time. This is in particular of interest when the sample contains particles which respond differently to different wavelength components, for instance when a particle is capable of emitting photoluminescence with different properties dependent on the nature of the particle. This effect can also be produced by the use of more than one type of light source which have different wavelength characteristics, preferably in combination with a monochromatic device.
In many preferred embodiments of this invention, electromagnetic radiation, such as UV or visible light is transmitted onto the sample, in order to give rise to photoluminescence, in a set-up where the light source, the sample compartment and the detection elements all are situated approximately on the same axes, preferably where the sample compartment is situated between the light source and the detector elements. Surprisingly it was found that under these conditions it was possible to remove substantially all the excitation light which was transmitted through the sample by means of filters, even in situations where high amounts of energy were used for the excitation. Further, in many preferred embodiments of this invention it was found that it was possible to increase the efficiency of the electromagnetic radiation used for excitation by placing a reflecting device between the sample compartment and the detector which could reflect at least a portion of the energy transmitted through the sample compartment back towards the sample compartment, preferably where at least one of the surfaces which define the sample compartment was reflecting, preferably this reflecting device is one which has different reflectance properties at different wavelengths, preferably in such a way that it is substantially transparent to the photoluminescence signal. One such reflecting device is a dichroic mirror.
It is often preferable to use one or several state of the art image processing techniques, such as 2 dimensional filtering or image identification, to assess the number of particles, or any morphological property of a particle.
As mentioned above, it is a particular feature of the invention that compared to traditional microscopy methods, the enlargement is from relatively small to very small. Thus, it is often preferred that the spatial representation exposed onto the array of detection elements is subject to such a linear enlargement that the ratio of the image of a linear dimension on the array of detection elements to the original linear dimension in the exposing domain is smaller than 4:1.
The above-mentioned ratio is normally in the range between 3:1 and 1:100, preferably in the range between 2:1 and 1:100. In many practical embodiments, the ratio will be in the range between 2:1 and 1:2. It can be interesting, in particular with small high precision detection elements, to work with very small ratios, such as in the range between 1.4:1 and 1:100, e.g., in the range between 1:1 and 1:100.
Another way of expressing the ratio at which the image should preferably be formed on the array is to consider the imaging of the individual somatic cell on the detection elements. It is often preferred that the somatic cells are imaged on at the most 25 detection elements, in particular on at the most 16 detection elements and more preferred at the most 9 detection elements. It is even more preferred that the individual particles the parameter or parameters of which is/are to be assessed are imaged on at the most 5 detection elements, or even on at the most 1 detection element. The larger number of elements per particle will provide more information on the individual particles, while the smaller number of elements per particle will increase the total count that can be made in an exposure.
As mentioned above, it is one of the characterising features of the present invention that a relatively large volume of sample can be exposed to the detection array. The sample is contained in the interior of the domain or sample compartment, which normally has an average thickness of between 20 xcexcm and 2000 xcexcm, usually between 20 xcexcm and 1000 xcexcm and in many practical embodiments between 20 xcexcm and 200 xcexcm. Normally, the domain or sample compartment has dimensions, in a direction substantially parallel to the array of detection elements, in the range between 1 mm by 1 mm and 10 mm by 10 mm, but it will be understood that depending on the design, it may also be larger and, in some cases, smaller.
The volume of the liquid sample from which electromagnetic radiation is exposed onto the array is normally in the range between 0.01 xcexcl and 20 xcexcl, preferably in the range between 0.04 xcexcl and 4 xcexcl.
As mentioned above, the sample is preferably at stand still during the exposure. However, in another embodiment, the sample in the domain or sample compartment is moved through the domain or sample compartment during the exposure, and the exposure is performed over a sufficiently short period of time to substantially obtain stand still condition during the exposure. In either case, there is a close control of the volume of the sample from which the exposure is made, which is one very preferred feature of the present invention.
When at least a major part of the electromagnetic radiation emitted from the sample during exposure originates from or is caused by electromagnetic radiation supplied to the sample from a light source, it is highly preferred that at least a major part of the radiation from the light source has a direction transverse to the wall of the sample compartment or a plane defined by the domain, such as substantially perpendicular to the plane defined by the domain (or an increment plane if the compartment wall is curved), or between perpendicular and 10 degrees, preferably between perpendicular and 20 degrees, more preferably between perpendicular and 30 degrees and still more preferably between perpendicular and 45 degrees. This is in contrast to the case where the radiation enters from an edge, parallel to the plane of the sample compartment, which is considered highly disadvantageous as it will, for many sample types, give rise to sufficient illumination of only small rim part of the sample.
As mentioned above, the size of the volume is suitably adapted to the desired statistical quality of the determination. The size of the volume of the liquid sample is preferably sufficiently large to allow identification therein of at least two somatic cells. More preferably, the size of the volume of the liquid sample is sufficiently large to allow identification therein of at least four somatic cells. This will correspond to a repeatability error of approximately 50%. Still more preferably, the size of the volume of the liquid sample is sufficiently large to allow identification therein of at least 10 somatic cells. This will correspond to a repeatability error of approximately 33%. Even more preferably, the size of the volume of the liquid sample is sufficiently large to allow identification therein of at least 50 somatic cells. This will correspond to a repeatability error of approximately 14%. Evidently, where possible, it is preferred to aim at conditions where the size of the volume allows identification of even higher numbers. Thus, when the size of the volume of the liquid sample is sufficiently large to allow identification therein of at least 100 somatic cells, it will correspond to a repeatability error of approximately 10%, and when the size of the volume of the liquid sample is sufficiently large to allow identification therein of at least 1000 somatic cells, it will correspond to a repeatability error of as low as approximately 3%.
Expressed in another, more specific manner, one main aspect of the present invention is defined as a method for the assessment of the number of somatic cells in a volume of a liquid milk or milk product analyte material, the method comprising
arranging a volume of between 0.01 xcexcl and 20 xcexcl of a liquid sample representing the liquid analyte material in a sample compartment having a wall part defining an exposing area, the wall part allowing electromagnetic signals from the sample in the compartment to pass through the wall and to be exposed to the exterior,
exposing, onto an array of active detection elements, an at least one-dimensional spatial representation of electromagnetic signals having passed through the wall part from the sample in the sample compartment, the representation being one which is detectable as an intensity by individual active detection elements, under conditions which will permit processing of the intensities detected by the array of detection elements during the exposure in such a manner that representations of electromagnetic signals from somatic cells are identified as distinct from representations of electromagnetic signals from background, the conditions involving such a linear enlargement that the ratio of the image of a linear dimension on the array of detection elements to the original linear dimension in the exposing domain is between 3:1 and 1:100, and such that individual somatic cells are imaged on at the most 25 detection elements of the array of detection elements,
the sample in the sample compartment being at stand still or substantially at stand still during the exposure, and at least a major part of the electromagnetic radiation emitted from the sample during exposure originating from or beings caused by electromagnetic radiation supplied to the sample from a light source, at least a major part of the radiation from which has a direction which is transverse to the wall of the sample compartment,
processing the intensities detected by the detection elements in such a manner that signals from somatic cells are identified as distinct from background signals,
and correlating the results of the processing to the number of somatic cells in a volume of the liquid analyte material.
As mentioned above, the signal which is detected by the detecting elements originates from one or several types of molecules of types which bind to, are retained within, or interact with, the somatic cells, such molecules being added to the sample or the isolated particles before or during exposure, the molecules being molecules giving rise to one or several of the following phenomena: attenuation of electromagnetic radiation, photoluminescence when illuminated with electromagnetic radiation, scatter of electromagnetic radiation, raman scatter. In the presently most preferred embodiments, an effective amount of one or more nucleic acid dyes and/or one or more potentiometric membrane dyes is added.
The duration of the exposure is normally in the range from 100 milliseconds to 5 seconds, in particular in the range of 0.5 to 3 seconds. The exposure may be performed as multiple exposures before the intensities detected by the detection elements are processed, but it is normally preferred that the exposure is performed as a single exposure.
A number of further embodiments and variants of the invention are claimed in claims 40-65 and are discussed later in the present description. Important embodiments of the invention appear from the figures and examples which follow, and the following detailed description of embodiments further elaborates on the invention and closely related subject matter.